This invention relates to a hyraulic control system for a vehicle having an automatic gear transmission in which changes in drive ratio are controlled jointly by load demand on a super charged vehicle engine and vehicle speed.
A hydraulic transmission control system comparable to that of the present invention is already known wherein the drive ratio shift points are predetermined by engine load and vehicle speed. A throttle pressure valve connected in series with a main pressure valve to the engine driven pump produces an operating pressure as a function of engine load independently of the position of the drive ratio selection lever of the transmission. A cam angularly displaced by a pull exerted on a cable to a position corresponding to that of the fuel or gas pedal, causes piston displacement in the throttle pressure valve thereby prestressing a valve seating spring in accordance with engine load to produce a throttle pressure. A modulation valve connected in parallel with the throttle pressure valve to the pump through the main pressure valve in series therewith is also biased to a seated position and is subjected to the operating pressure as well as the throttle pressure. The operating pressure is derived from the pump through a main pressure valve. The modulation pressure output of such modulation valve in most cases is greater than the throttle pressure, in accordance with the ratios of the valve and piston pressure surfaces and the changes in the operating pressure as a function of the engine load, to influence drive ratio change or shift and determine the pressure in the servo pistons during the drive ratio shifting process.
The pressure dependent on vehicle speed of travel, is produced by a governor valve on the driven transmission shaft and lockout valves which prevent downshift from speed ratios corresponding to the positions of the aforementioned drive ratio selection lever. Furthermore, upshift to speed ratios above such selected speed ratios are also prevented.
Hydraulic control systems associated with the foregoing type of automatic gear transmission have proven to be practical with all engines developing suction pressure for the determination of drive ratio switching points and operation of control elements in dependence on engine load and speed of travel by means of the throttle pressure valve and the regulator valve. In such transmissions, engagement and disengagement of drive establishing couplings and brakes are effected without discomfort to the driver and the passengers of the vehicle.
In the case of vehicles driven by a super charged engine, the foregoing control operations of the automatic transmission are not satisfactory because engine torque is not alone reflected by the position of the fuel control gas lever or pedal. It is therefore an important object of this invention to provie a hydraulic control system for gear transmissions which may also be used for transmissions driven by super charged engines with minimum structural or control modification.